Yup, October has, in the past 5 years, become one of DLR's most popular months. That being said, most Mondays-Wednesdays are still quiet and have low crowds (Columbus Day monday might have slightly higher crowds). Mickey's halloween Party is on Tuesdays and Fridays. I don't know how it affects crowds on Tuesdays, but I know that on Fridays it pushes tons of people out of DL and into DCA and DCA got REALLY crowded.
The Halloween decorations and offerings have really improved over the past few years and thus more people are coming on weekends. Unfortunately for the crowd haters, the weekends in October are going to be the only times when they do fireworks and Fantasmic.
All that being said, I would still advise you to go during this time. It's an awesome time to visit. Weather is great and the Halloween stuff is all really fun. And if you have a rough general plan, you can manuever around crowds and still have an amazing experience!